Emerging hybrid imaging systems in which magnetic resonance (MR) imaging is combined with positron emission tomography (PET) have recently attracted interest for a variety of clinical indications including oncology. Whilst PET provides functional information with high sensitivity, the superior soft tissue contrast of MR gives excellent anatomical localization. However, in PET imaging systems the necessity arises to generate attenuation maps that describe the attenuation of the 511 keV gamma photons created by positron annihilation.
Conventionally such attenuation maps are generated from a computed tomography (CT) image since the Hounsfield units of x-ray radiation attenuation may be mapped directly to gamma photon attenuation values. Frequently, CT and PET imaging are combined in a hybrid PET/CT imaging system in order to generate such attenuation maps. However the integration of a CT imaging system with a hybrid PET/MR imaging system presents numerous issues relating to integration, compatibility and cost.
MR Dixon sequences have been investigated for the generation of PET attenuation maps. Using Dixon techniques, soft tissue can be readily separated into its water and fat components. A document “Tissue classification as a potential approach for attenuation correction in whole-body PET/MRI: evaluation with PET/CT data”, Journal of Nuclear Medicine 50(4) (2009) 520-526; by Martinez-Moeller, A. et al, reports that with moderate additional effort, attenuation maps with four compartments (background, lung tissue, muscle and connective watery tissue, and fatty tissue) can be generated. However, cortical bone tissue, such as the vertebrae, is not visible in standard MR sequences and is commonly ignored during attenuation correction (AC) which can lead to substantial errors especially for metastases located close to bone.
A document “MRI-Based attenuation correction for hybrid PET/MRI systems: A 4-class tissue segmentation technique using a combined ultrashort-echo-Time/Dixon MRI sequence”, Journal of Nuclear Medicine 53(5) (2012) 796-804, by Berker, Y. et al reports that ultrashort echo time (UTE) sequences have also been successfully combined with Dixon imaging to include bone tissue in AC of the head. However, UTE is still prone to imaging artefacts and long scan times with respect to whole-body applications.
Image processing techniques have also been used to segment vertebrae from MR images. An intensity based approach is reported in document “Atlas-based segmentation of degenerated lumbar intervertebral discs from MR images of the spine”, IEEE Transactions on Biomedical Engineering 56(9) (2009) 2225-2231, by Michopoulou, S. et al. Conventionally such intensity-based approaches use a single MR image for vertebra detection, most commonly a T1- or a T2-weighted MR image.
However, the above approaches to generation of PET attenuation maps suffer variously from poor segmentation of cortical bone, or from the drawbacks of increased acquisition times consequent to the need to acquire additional imaging data from UTE, T1 or T2-weighted images.